This invention relates to a belt unit having an endless belt stretched around a plurality of rollers, and an image forming apparatus using the belt unit.
Conventionally, there is a type of image forming apparatus that uses an endless belt as a conveyor belt that conveys a recording medium, a transfer belt that bears a toner image, or the like. Such an image forming apparatus is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2002-60039 (particularly, in pages 4 to 5 and FIGS. 5 to 6).
FIG. 17 shows an example of a conventional belt unit used in an image forming apparatus. The belt unit includes a drive roller 205 having a surface layer made of a high friction material, an idle roller 206 provided substantially in parallel to the drive roller 205, and an endless belt 204 stretched around the drive roller 205 and the idle roller 206. The drive roller 205 is rotatably supported by a belt frame 208 via bearings 202. The idle roller 206 is rotatably supported by the belt frame 208 via bearing members 210L and 210R.
Positioning members 212a and 212b and positioning members 213a and 213b are provided in the image forming apparatus. The positioning members 212a and 212b engage shaft portions 206L and 206R of the idle roller 206 to position the idle roller 206 in the vertical direction. The positioning members 213a and 213b engage bearings 202 (one of which is shown in FIG. 17) of the drive roller 205 to position the drive roller 205 in the front-rear direction.
As schematically shown in FIGS. 18A and 18B, if the parallelism between the drive roller 205 and the idle roller 206 is low, it is known that the endless belt 204 (proceeding in the direction indicated by P) skews as indicated by an arrow +S (FIG. 18A) or an arrow −S (FIG. 18b) according to the inclination of the idle roller 206 with respect to the drive roller 205. As shown in FIG. 19, in order to prevent skewing of the endless belt 204, a pulley 209 is fixed to an end of the idle roller 206 in the axial direction. A bead 215 is bonded to an inner surface of the endless belt 204, and engages the groove of the pulley 209.
However, if the positioning accuracies of the positioning members 212a, 212b, 213a and 213b (FIG. 17) are low, or if the image forming apparatus is distorted because of lack of rigidity, the belt frame 208 may deform, and therefore the parallelism between the drive roller 205 and the idle roller 206 may be reduced. In such a case, due to the skewing of the endless belt 205, the bead 215 is strongly pressed against the groove of the pulley 209, and therefore a large shear stress may be applied to the bonding portion of the bead 215 and the endless belt 204. Accordingly, the bead 215 may be peeled away from the endless belt 204.